November 19, 2009

1,1-dicyano-2-methoxy-2-(1′-naphtylmethyl)-ethylene

This procedure employs methyl triflate. The final product is a strong irritant. See the notes below.

A slurry of 1-naphtaleneacetic acid 26.00g (140 mmol) in dichloromethane 60mL in a 250mL round flask was placed on ambient water bath and neat oxalyl chloride 13.0mL (150 mmol) was added followed by 5 drops of DMF (not more, else the gas evolution commences too fast). The flask was equipped with a Drierite-filled gas outlet tube and stirred on ambient bath overnight (8h30 min, the gas evolution ceased and the mix became homogennous). The flask was equipped with a vacuum distillation shortpath adaptor, placed on oil bath and the solvent was distilled off at atmospheric pressure from a 80C bath. The residue was then vacuum-distilled at 0.4-0.5 Torr from 80C to 135C bath, the pure acyl chloride distilled at 110C/0.45 Torr. Y=26.11g of a light orange-yellow oily liquid (91%Y)

60% NaH in mineral oil, 5.0g (125 mmol) was loaded into Ar-flushed 1L round flask with a large stirbar. Anhydrous THF 0.5 L (approx) was added via canula and the slurry was cooled on ice bath. Neat CH2(CN)2 4.40g (66.5 mmol) was added in one portion (gas evolution!) [1] and the mix was stirred on ice for 30 min. An addition funnel was charged with benzene 20mL and 1-naphtylacetyl chloride 12.00g (58.63 mmol) and this solution was then dropwise added into vigorously stirred reaction mix on ice bath under Ar (gas evolution) over a 30 min period. The addition funnel was washed with benzene 10mL and the washings were also added to the reaction mix. 10 min after complete addition, the cooling bath was removed and the reaction mix was stirred at RT overnight (7h30min). The reaction mix was then briefly sonicated (1 min) to break few remaining unreacted chunks of NaH, then carefully evaporated to dryness. The obtained foamy residue containing the Na-enolate salt intermediate was suspended in anh MeCN 0.5L (added via canula) and then stirred under Ar and occasionally sonicated over a 30 min period until all chunks disappeared and a cloudy solution formed. The mixture was placed on ice bath and after 20 min on ice, neat methyl triflate [2] 9.0mL (81.5 mmol) was added by a syringe dropwise over a 10 min period. After additional 10 min stirring on ice, the cooling bath was removed and the mixture was stirred at RT under Ar overnight (15h30 min). At this point the TLC and HPLC indicated disappearance of the acylmalononitrile intermediate and a formation of two products in about 3:1 ratio. The reaction was quenched by addition of triethylamine 7.0 mL, stirred for 15 min and then evaporated to dryness. The residue was suspended in toluene (100mL), the slurry was filtered through a medium-porosity glass Buchner funnel (150mL size), the insoluble material was thoroughly washed with additional toluene (150mL) and discarded. The filtrates were evaporated and the residue was dissolved in a mix of DCM and toluene (20+20mL), the mix was applied onto a column of silica (250g) in straight DCM and eluted with DCM. The pure methoxy product migrated first and separated nicely from the sideproduct on silica in DCM. Y=9.200g (63%) of a pale yellow sticky goo that gradually solidified on highvac into a hard white crystalline mass. The product is a potent irritant [3].

Note 1: It is better to add solid malononitrile rather than a warm molten liquid – the liquid tends to solidify in the syringe

Note 2: Methyl triflate is a supremely toxic and dangerous alkylating agent as discussed previously. Use nitrile gloves throughout, keep the triflate in the hood at all times, treat the rotovap and the rotovap waste with a great care until decontaminated with ammonia.

Note 3: The product is completely odorless but a rather creepy irritant – the structure is closely related to CS tear gas commonly used by the police and military. Use gloves, do not spill it outside hood, do not breath vapors (= swollen itchy nose for hours afterwards; certizine pills are a pretty useful thing to have at hand). For the love of God wash your hands with a soap before going to bathroom. The material is best used right away as a goo – before it solidifies – crushing a hard crystalline solid is no fun with a strong irritant.

There is an old chemist joke, about washing hands twice – before and after. But I met a peptide chemist who actually had an accident in his younger years: he cleaved peptide stuff from Merrifield resin with anhydrous HF and then purged away HF but apparently some dew of diluted HF remained on the apparatus somewhere and he got it on his fingers and did not even notice. (Diluted HF burns insidiously and by the time one feels that something is wrong the damage is already done, it gets very painful later on and it heals rather slowly.) He resorted to sleeping with a jar filled with water placed in between the sofa pillows, to immerse his burned parts for the first few nights after the incident, so that he could get some sleep.
So in the case of HF, it should be “For the love of God and everything that is holy”

Someone left me a comment a while back about a fellow who spilled P-32 on his hands and how he’s been urinating in lead gloves for several months after this.

Did you have to use methyl triflate? The enolate must be pretty stable, why not heat it up with something less reactive and less nasty, perhaps with something bigger, too, that does not produce a tear gas essence?

Also, I’ve never distilled my acid chlorides. If you’re doing a column anyway, why bother?
I have distilled thionyl chloride though, in my first year of college. I came to the lab one day and thought “hmmm, all my acid chlorides turn up brown – must be some crap in thionyl chloride. It shouldn’t be yellow. Let’s distill it.” I did it alone, and it was surprisingly uneventful. Looking back, I don’t think I’d let myself do it. Needless to say, it didn’t help acid chlorides much.

Distillation of acyl chloride is probably unnecessary but I was making a big batch and I did not have the exact procedure for this prep, only general one with different substituent. Besides, 1-naphtaleneacetic acid takes on funny dark colors during the acylchloride formation so I wanted to be sure I have a nice starting material since I was usure how easily the products will purify later on. The acyl chloride is rather highboiling, I wished we had a big coffee-can Kugelorohr in our lab – something we discussed buying several times but never did. Alas I had to get by with a regular shortpath, it took couple of hours of internet surfing to distill everything over.

As for the choice of alkylating agent, the original procedure used reflux with dimethyl sulfate (5 equivs) in aqueous bicarbonate biphasic mix, and a similar procedure in patent lit used tons of TMS-diazomethane. One needs to use a very hot hard alkylating agent to get O-alkylation over C-alkylation product. I think the sideproduct I saw was actually C-alkylated product, fortunately it separated away quite nicely.

The intermediate was already in the Na enolate salt form from the acylation – I could have added the alkylating agent in there directly except that MeOTf has a nasty tendency to polymerize THF so I had to switch the solvent to MeCN. Too bad one cannot do NaH deprotonations in MeCN. I suppose a process chemist would run this in something like 2-methyl-THF or glyme with dimethyl sulfate in a one-pot sequence. This was my first trial and I needed a multigram quantity of the compound in hurry so I had no time to mess with the optimisation.

Maybe one could use dimethyl carbonate as a methylatintg reagent and a solvent in this step, first the NaH with malononitrile at RT in DMC, then reflux with added DBU as a catalyst, to bring about the O-methylation.